Figure 3. Structure of AcrIII-1 bound to cA₄.

(a) Superimposition of the apo SIRV1 gp29 structure (salmon) and in complex with cA₄ (blue), highlighting the movement of the loop and α-helix upon cA₄ binding. cA₄ is shown coloured by element. (b) Surface representation of the structure of SIRV1 gp29 (blue) in complex with cA₄, emphasising the complete burial of the ligand. (c) Surface representation of the apo structure of SIRV1 gp29 (salmon) with cA₄ in the position observed in the complex structure, indicating that the binding site is pre-formed. (d) Structure of cA₄ bound to SIRV1 gp29. The two active site histidine residues (modelled based on the position of the alanine side chain in the H47A variant crystallised with cA₄; coloured to represent residues from different monomers) are in suitable positions to act as the general acid, protonating the oxyanion leaving group. The corresponding ribose sugars have 2’-hydroxyl groups suitably positioned for in-line nucleophilic attack on the phosphodiester bond.